The lateral corticospinal tract is a relatively minor tract in lower mammals but is quantitatively and functionally very important in primates, particularly in humans, in which it contains more than 1 million axons. This number still represents a relatively small proportion of the outflow from the cortex because there are approximately 20 million axons in the cerebral peduncles. Nevertheless, the corticospinal pathway is critical for the fine independent control of finger movement, inasmuch as isolated lesions of the corticospinal tract typically lead to a permanent loss of this ability, even though other movement abilities are often recovered with such lesions. Indeed, in primates, corticospinal synapses directly onto motor neurons are particularly prevalent for the motor neurons controlling finger muscles and are probably the basis of the ability to make independent, finely controlled finger movements.
The corticobulbar tract, which projects to the cranial nerve motor nuclei, has subdivisions that are comparable with the lateral and ventral corticospinal tracts. For example, part of the corticobulbar tract ends contralaterally in the portion of the facial nucleus that supplies muscles of the lower part of the face and in the hypoglossal nucleus. This component of the corticobulbar tract is organized like the lateral corticospinal tract. The remainder of the corticobulbar tract ends bilaterally.
The rubrospinal tract originates in the magnocellular portion of the red nucleus, which is located in the midbrain tegmentum. These fibers decussate in the midbrain, descend through the pons and medulla, and then take up a position just ventral to the lateral corticospinal tract in the spinal cord. They preferentially affect motor neurons controlling distal musculature, as do the corticospinal fibers. Red nucleus neurons receive input from the cerebellum and from the motor cortex; thus making this an area of integration of activity from these two motor systems.
The Medial System
The ventral corticospinal tract and much of the corticobulbar tract can be regarded as medial system pathways. These tracts end on the medial group of interneurons in the spinal cord and on equivalent neurons in the brainstem. The axial muscles are controlled by these pathways. These muscles often contract bilaterally to provide postural support or some other bilateral function, such as swallowing or wrinkling of the brow.
Other medial system pathways originate in the brainstem. These include the pontine and medullary reticulospinal tracts, the lateral and medial vestibulospinal tracts, and the tectospinal tract.
The cells that give rise to the pontine reticulospinal tract are in the medial pontine reticular formation. The tract descends in the ventral funiculus, and it ends on the
CHAPTER 9 Organization of Motor Function ipsilateral medial group of interneurons. Its function is to excite motor neurons to the proximal extensor muscles to support posture.
The medullary reticulospinal tracts arise from neurons of the medial medulla, particularly those of the gigantocellularis reticular nucleus. The tracts descend bilaterally in the ventral lateral funiculus, and they end mainly on interneurons associated with cell groups of medial motor neurons. The function of the pathway is mainly inhibitory.